Image interlacing and joining in a printer

ABSTRACT

An ink-jet printer that prints using an interlace ratio of higher than 2:1. The print head has an internozzle spacing of N, while the interlace ratio n:1 is selected so that n is an integer divisor of N+2 or N+4 or is equal to N+1. Multiple interlaced images can be &#34;stitched&#34; together by printing a first interlaced image, moving the print head and printing a second interlaced image such that the head of the second image overlaps with the tail of the first interlaced image.

TECHNICAL FIELD

This invention relates to raster printers, and more particularly to anapparatus and method for interlacing ink drops forming printed scanlines from multiple nozzles onto a transfer surface or media to improveprint quality and minimize print artifacts and to join adjacentinterlaced images to form a larger interlaced image.

BACKGROUND ART

The field of ink-jet printing is replete with references describingsolutions to problems associate with placing ink drops on a printmedium. In particular, color ink-jet printing requires careful placementof ink drops to meet print resolution and color fidelity requirementswithout producing undesired printing artifacts.

Ink drop placement-related problems vary in severity with a large numberof printer-related variables including desired printing speed, printhead array configurations, transfer versus direct printing, aqueousversus phase change ink, the printing resolution required, printpostprocessing employed, if any, and the type of print medium employed.

Many prior print interlacing methods and print head nozzle arraypatterns are known because of the correspondingly wide variety of nozzlearray configurations, ink types, print media supports, print head andmedia movement mechanisms employed by ink-jet printers.

Line interlacing entails printing adjacent lines of dots of a particularcolor during sequential scans of the print head. For example, lines 1,3, 5, etc., are printed during first scan, and lines 2, 4, 6, etc., areprinted during the next scan.

Printing artifacts can also be caused by variations between the nozzleson the print head. The nozzles must behave similarly and consistently toproduce uniform shades and solid fills. Differences in drop mass, nozzleaim, and timing can result in degraded image quality.

Print head consistency can be improved by a process called"normalization." During normalization, the performance of each nozzle onthe print head is measured. An adjustable driver circuit is associatedwith each nozzle. As part of the normalization process, parameters ofthe individual driver circuits are modified to bring each nozzle'sperformance within an acceptable range.

SUMMARY OF THE INVENTION

The present invention provides a method and an apparatus that providesinterlaced printing at an increased interlace ratio.

Generally, the method of the present invention includes printing aninterlaced image such that bands in the printed image include theoutputs from at least three different nozzles. Any disparity inperformance between the different nozzles is effectively eliminated bythe visual averaging of their outputs.

At interlace ratios of 4:1 and 6:1 print head normalization and theconcomitant separate and individual driver voltages for each nozzle maybe eliminated.

In a preferred printer of the invention, a drum driver rotates the drumabout a drum axis, causing Y-axis, or scanning, motion. A singlerotation of the drum equals a single scan. A carriage servo moves aprint head along the length of the drum along the X-axis with a slewingmotion. The relative speeds of the drum rotation and carriage iscontrolled such that the print head lays down scan lines of ink drops onthe drum in an interlaced fashion such that there is the desired spacingbetween the adjacent scan lines of ink drops to achieve uniform colorshading and secondary color solid area fills without printing artifacts.

According to a further preferred method of the present invention, theinterlace ratio is divisible by two, enabling adjacent scan lines to bepaired, further decreasing print artifacts.

It is another feature of the present invention that the interlacingmethod is especially useful in imaging with solid or phase change inkprinting.

It is an advantage of the present invention that a method of interlacinga printed image is provided that removes the need to normalize the printhead during manufacturing, thereby saving time and cost in themanufacturing process.

It is another advantage of the present invention that uniform shades andsolid area fills are achieved in printing without undesirable printingartifacts, such as banding.

These and other aspects, features and advantages of the presentinvention are obtained by the apparatus and method of using thatapparatus by laying down scan lines of ink drops in an interlacedfashion that improves print quality.

BRIEF DESCRIPTION OF THE DRAWINGS

The aspects, features and advantages of the invention will becomeapparent upon consideration of the following detailed disclosure of theinvention, especially when it is taken in conjunction with theaccompanying drawings wherein:

FIG. 1 is a general block diagram illustrating a printer made accordingto the present invention;

FIG. 2 is a perspective view of an exemplary print head and associatedtransfer drum of the printer of FIG. 1;

FIG. 3 is an enlarged face view of a portion of the print head of FIG.2;

FIG. 4 is an illustration of the printing method of the printer of FIG.1, showing the impact locations of one or more nozzles in the samecolumn for adjacent columns of nozzles during consecutive rotations ofthe transfer drum, with an internozzle spacing of 10 scan lines and a3:1 interlace ratio;

FIG. 5 is an illustration of the printing method of the printer of FIG.1, showing the impact locations of one or more nozzles in the samecolumn for adjacent columns of nozzles during consecutive rotations ofthe transfer drum, with an internozzle spacing of 10 scan lines and a4:1 interlace ratio;

FIG. 6 is an illustration of scan line pairing;

FIG. 7 is an illustration of stitching or interleaving together twointerlaced images having an internozzle spacing of 10 scan lines and a4:1 interlace ratio;

FIG. 8 is a simplified illustration of an interlaced image showing itshead and tail; and

FIG. 9 is a simplified illustration of stitching or interleavingtogether two component interlaced images to form a combined interlacedimage.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, an exemplary printer 20 made according to thepresent invention is shown. Printer 20 receives scan data from a datasource 22. This data defines the colors to be printed at each pixellocation on a predetermined image area of a print medium.

A printer driver 24 within printer 20 receives the data and, inresponse, controls operation of print engine 26. Control includesfeeding formatted data to a print head 28, the movement of which isprovided by a carriage 30, shown in FIG. 2, controlled by a carriageservo 32. A motor 34 rotates transfer drum 36 about an axis 38. Motor 34is controlled by a motor controller 40 receiving control data fromprinter driver 24.

Print head 28 prints an image on transfer drum 36. When the image for asheet of print medium (not shown) is fully printed on transfer drum 36,the sheet is brought in contact with transfer drum 36, transferring thedeposited image to the sheet.

The image receiving portion of transfer drum 36 may be wider than arrayof nozzles on print head 28.

FIG. 3 illustrates a portion of a face 42 of print head 28 havingparallel columns 44 of ink jet nozzles 46. Each column 44 includes ablack, cyan, yellow, and magenta ink jet nozzle 46. Columns 44 arearranged in line with the relative scanning motion between print head 28and transfer drum 36, allowing ink droplets from a single column 44 tooverlay each other during a single scan.

The internozzle spacing N between adjacent columns 44 as seen in FIG. 3may be selected based on criteria not relevant to the present invention.These criteria includes the minimum size of the nozzles and supportingstructure, desired print resolution, print speed, and cost. As describedbelow, certain internozzle spacings N provide a wider range of possibleinterlace options and thus may be preferred.

The internozzle spacing N dictates the number of scans, and thusrotations of transfer drum 36 that must occur, to print a completeimage. A print head 28 having an internozzle spacing of N requires Nscans with N-1 respective steps between the scans.

In practice, carriage servo 32 moves print head 28 a constant slew speedwhile transfer drum 36 rotates. With 300 scan lines per inch (118 scanlines per centimeter) and an interlace of 6:1, this results in an imageskewed by 0.02 inches (78.7 μm) over its complete length. Smallerinterlace ratios and higher resolutions result in decreased skewing ofthe image.

Because even slight offsets or inconsistencies in the placement ofadjacent scan lines results in readily noticeable print artifacts,carriage servo 32 must be able to precisely locate print head 28.

FIG. 4 illustrates the method of practicing the present invention withprint head 28 having an internozzle spacing N of 10 scan lines and a 3to 1 (3:1) interlace ratio. The row 50 of adjacent symbols representsthe adjacent scan lines that are printed by print head 28. All symbolshaving the same shape and orientation represent scan lines that areprinted by the same column 44 of nozzles 46 of FIG. 3. The number withineach symbol represents the number of the scan during which the scan lineis printed.

Therefore, scan lines 52-61 are printed by the same column of nozzles onconsecutive rotations of transfer drum 36. Similarly, scan lines 62-71are printed by another column of nozzle. Furthermore, scan line 52 isprinted at the same time as scan line 62. The column of nozzles thatprints scan lines 62-71 is adjacent to the column of nozzles that printsscan lines 52-61. Thus, the distance between scan line 52 and scan line62 is equal to the internozzle distance or spacing N, see FIG. 4.

An alternative representation of this interlace pattern is the numericstep sequence of print head 28 between scans. For the 3:1 interlace ofFIG. 4, the step pattern is 3, 3, 3, 3, 3, 3, 3, 3, 3.

FIG. 4 shows that the 3:1 interlaced image has bands 75 that arecomposed of the scan lines from three different nozzles 46. Bands 75have a width equal to the internozzle spacing N. The larger the numberof nozzles involved in making any small region of the image the less thesensitivity to variation in dropmass from the nozzles, and thus lessprinting artifacts.

FIG. 5 illustrates the method of practicing the present invention withanother print head 28 having the same internozzle spacing N of 10 scanlines but using a 4:1 interlace ratio.

The step pattern for this interlace pattern is 4, 4, 4, 4, 5, 4, 4, 4,4. Thus, all steps are the same except for the middle step, which is onegreater than the others. This extra displacement or movement of the scanline must occur when print head 28 is not printing on transfer drum 36,which occurs once per rotation of transfer drum 36.

With a 4:1 interlace ratio, each band of the interlaced image iscomposed from the output of four different nozzles. This increasedinterlace ratio further reduces the sensitivity to variation in dropmassbetween nozzles.

Furthermore, because a 4:1 interlace ratio has an even number for itsstep, scan line pairing can be used to further improve image quality.That is, the first one-half of the scans results in every other scanline being deposited. The middle step is increased by one, from 4 to 5,so that the print head 28 can print the remaining scan lines. Byadjusting the middle step to a value slightly different from an integerscan line distance, all subsequently printed scan lines will not beequidistant from both of their adjacent scan lines, but rather movedcloser, or "paired," with one of their adjacent scan lines. This pairingresults in a noticeable decrease in printing artifacts.

FIG. 6 shows a simplified view of scan line pairing. Scan lines 90represent the first set of scan lines to be printed. Scan lines 92represent the second set of interlaced scan lines to be printed. It hasbeen experimentally determined that a distance of 0.8 to 0.5 pixelwidths between paired scan lines (where the distance between a scan lineand the second following scan line is 2 pixel widths) or the second setof scan lines are offset an additional 0.2 to 0.5 pixel widths from thepixel integer position is preferred. The pixel integer position is theposition midway between the 2 pixel widths separating the scan line andthe second following scan line.

Scan line pairing of interlaced images is addressed in more detail in aco-pending application Ser. No. 08/381,615, entitled "Pairing of InkDrops on a Print Medium," filed Jan. 30, 1995, which is herebyincorporated by reference, and which has a common assignee.

The examples of FIGS. 4 and 5 have an internozzle spacing N of 10. Thisspacing was intentionally chosen this small to ease the graphicalillustrating of the interlace pattern. Due to size constraint of printhead 28, a typical internozzle spacing is significantly larger.

One typical implementation of print head 28 has an internozzle spacing Nof 28, and prints at a resolution of 300 dots per inch. A 4:1 interlacecan be implemented with a stepping pattern of 4, 4, 4, 4, 4, 4, 6, 4, 4,4, 4, 4, 4, 5, 4, 4, 4, 4, 4, 4, 6, 4, 4, 4, 4, 4, 4.

The same print head 28 having an internozzle spacing N of 28 canimplement a 6:1 interlace using a stepping pattern of 6, 6, 6, 6, 6, 6,6, 6, 6, 6, 6, 6, 6, 7, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6.

A different print head 28 having an internozzle spacing N of 22 canprint a 4:1 interlaced image with a stepping pattern of 4, 4, 4, 4, 4,4, 4, 4, 4, 4, 5, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4.

A preferred value of interlace ratio n₁, can be selected where n₁, is aninteger divisor of N+2, where N is the internozzle spacing. Thus, if Nis 28, then N+2 is 30 and n₁ can be any of 2, 3, 5, 6, 10, 15. If N is22, then N+2 is 24 and n₁ can be any of 2, 3, 4, 6, 8, 12. Thesepossible interlace values require the center step of their respectivestepping patterns to be increased by only one. The N=10, 4:1 interlaceexample given above, as well as the N=28, 6:1 interlace, and the N=22,4:1 interlace are all examples of preferred values n₁. Less desirable,but still acceptable, values of possible interlace ratio n₂ can beselected where n₂ is an integer divisor of N+4. Thus, if N is 28, thenN+4 is 32 and n₂ can be either 4 or 8. If N is 22, then N +4 is 26 andn₂ can only be 13. Ratio values n₂ are less desirable than those ratiovalues n₁ in that values n₂ require at least one step in the steppattern that is two greater than n₂. The N=28, 4:1 interlace examplegiven above is an example of an interlace ratio n₂.

Yet another possible interlace ratio n₃ is possible where n₃ is N+1.Such a high interlace ratio can require relatively large movements ofprint head 28 for large internozzle spacing N.

Increasing the interlace ratio n above 2:1 has the effect of decreasingprinting artifacts caused by disparities in the dropmass of adjacentnozzles. Increasing the number of nozzles that participate in creating aportion of an image decreases the sensitivity to any one nozzle.However, if the interlace ratio gets too large, the spatial bandfrequency becomes sufficiently small to become noticeable, detractingfrom image quality. With 300 dots per inch resolution (118 dots cm-1),optimum interlace ratio appears to be either 4:1 or 6:1.

Care must be taken when printing the edges of an interlaced image whenthe interlace ratio n is greater than two. FIG. 7 illustrates the edgesof an interlaced image where the internozzle distance N is 10 and theinterlace ratio n is 4:1.

Row (A) illustrates the results of printing using all of the columns 44of nozzles 46 on print head 28. Each circle represents a scan line ofthe image, with the numbers contained in the circles indicating thenumber of the column 44 that prints the ink drops that form the scanline. Print head 28 slews in the X-axis direction during printing. Asdrawn, scan lines containing a "1" are the printed by the leftmostcolumn 44 of nozzles 46 printing the interlaced image. Scan lines 100,101, 102, and 103 print during the first scan; scan lines 104, 105, 106,and 107 print during the second scan, and so on. The majority of theinterlaced image extends to the right of the scan lines illustrated inRow (A).

The resulting interlaced image has gaps 110 in it and is not fullyfilled until scan line 112. The print region containing gaps 110 at thebeginning of printing is termed a "head" 120 for the purposes ofdiscussion in this document. Gaps 110 in head 120 would normally beconsidered to be unacceptable in a printed image. To avoid the gaps, theleftmost edge of the interlaced image is mapped to scan line 112; noscan lines to the left of scan line 112 are then printed.

At the other end of print head 28 a similar situation can occur,resulting in a "tail" 130 as seen still in FIG. 7. Row (B) illustratestail 130, with the majority of the interlaced image extending to theleft. To avoid gaps 132 in tail 130, the rightmost edge of the interlaceimage cannot go beyond scan line 134.

FIG. 8 shows a simplified view of an interlaced image along the X-axis,showing the fully filled region 142. The head 144 and tail 146 aresimply represented as ramps. By confining the output of print head 28 tobetween scan lines 148 and 150, the gaps can be avoided.

Returning to FIG. 7, it can be seen that gaps 110 of head 120 perfectlymatch the printed scan lines in tail 130. A "combined" interlaced imagemay be interleaved or "stitched" together from two "component"interlaced images by overlapping their respective head 120 and tail 130.

FIG. 9 shows a simplified representation of stitching together twocomponent interlaced images to result in a wider combined interlacedimage. Printing a combined interlaced image substantially wider thanprint head 28 can be achieved according to the present invention byprinting a first component interlaced image 160 having a tail 162,moving print head 28 to the beginning 163 of the tail 162 and thenprinting a second component interlace image, starting with its head 166.The resulting combined image can be mapped between a first scan line 168in the first component image 160 and a second scan line 170 in thesecond component image 164. The combined image need not be the fullwidth available between first and second scan lines 168, 170.

It will be recognized that the column of nozzles that prints the head166 of the second component interlaced image 164 need not be adjacent toan end of print head 28. If the width of the desired combined interlacedimage is less than twice the maximum width of a single componentinterlaced image printable by print head 28, then print head 28 needonly be moved enough between prints of component images to accommodateprinting the edge of the combined image. Not all columns 44 of nozzles46 will be used in printing the second component interlaced image 164.If desired, both the first and second component images may be decreasedin width such that their head and tail overlap in the center of theimage.

Furthermore, it will be recognized that a combined interlaced image cancomprise more than two component interlaced images.

The terms and expressions which have been employed in the foregoingspecification are used therein as terms of description and not oflimitation, and there is no intention, in the use of such terms andexpressions, of excluding equivalents of the features shown anddescribed or portions thereof, it being recognized the scope of theinvention is defined and limited only by the claims which follow.

What is claimed is:
 1. A method for printing an interlaced image using aprint head having a plurality of nozzles, each pair of adjacent nozzleshaving a predetermined internozzle spacing of N pixel widths, the methodcomprising the steps of:(a) printing a plurality of sets of scan linesin a Y-axis direction on a rotating receiving surface, the plurality ofsets of scan lines including at least a first set of scan lines and asecond set of scan lines; and (b) interlacing said plurality of sets ofscan lines on the rotating receiving surface in an X-axis directionwithin a single print band by use of an interlace ratio n₁ that is aninteger divisor of the internozzle spacing N+2.
 2. The method of claim1, further comprising a step of printing a third set of scan lines,wherein said third set of scan lines is interlaced with said first andsecond sets of scan lines within a single band.
 3. The method of claim2, further comprising a step of printing a fourth set of scan lines,wherein said fourth set of scan lines is interlaced with said first,second and third sets of scan lines within a single band.
 4. A methodfor printing an interlaced image on a print medium, said methodperformed by an ink-jet printer having a print head and a print mediumthat undergo relative movement, the print head having a plurality ofprint nozzles spaced apart by N pixel widths, said method comprising thesteps of:(a) printing a first set of scan lines using the plurality ofprint nozzles; (b) moving the print head relative to the print medium adistance of n pixel widths, where n is greater than two; (c) printing asecond set of scan lines interlaced with the first set of scan lines ina scanning direction using the plurality of print nozzles; (d) movingthe print head relative to the print medium a distance of n pixelwidths, where n is greater than two; and (e) repeating steps (a) through(d) until the interlaced image is completed.
 5. The method of claim 4,wherein n is an integer divisor of N+2.
 6. The method of claim 4,wherein n is an integer divisor of N+4.
 7. The method of claim 4,wherein n is N+1.
 8. The method of claim 4, wherein n is an even number.9. The method of claim 8, wherein pairs of adjacent scan lines in theinterlaced image are printed at spacings within the range of 0.8 to 0.5pixel widths.
 10. The method of claim 9, further comprising a step ofmoving the print head an additional distance of 0.2 to 0.5 pixels afterone-half of the interlaced image is printed.
 11. The method of claim 4,wherein steps (b) and (c) occur simultaneously.